1. Field of the Invention:
This invention relates to an automatic solar lighting apparatus for automatically directing the sunlight to a prescribed portion by use of a solar following sensor.
2. Description of the Prior Art:
Many attempts have been made to exploit effectively the sunlight for various purposes by use of a solar lighting system. In the solar systems, there are two types for exploiting the sunlight: a solar direct-receptive type for use in a solar system such as a solar power generator using a solar cell panel, and a solar indirect-lighting type for reflecting and directing the sunlight to a specific portion to be lit by use of a reflecting means.
The conventional solar system of the former types can be easily realized with a simple mechanism by which the solar cell panel is controlled merely to face the sun. On the other hand, the latter type is available for various uses because the portion to be lit can be freely determined, while it suffers a disadvantage that the reflecting means must be controlled with a high accuracy so as to equalize the angles of incidence and reflection of the sunlight striking the reflecting means. Thus, since the conventional solar system of this type calls for a complicated control device for changing the angle of reflection of the sunlight relative to the reflecting means in accordance with the sun which moves continuously in the sky, it inevitably turns out to be expensive and susceptible to mechanical troubles during service. Though there is a possibility of utilizing availably the sunlight for supplementary illumination of a room, indoor plant culture or the like, the conventional solar lighting systems have not been of practical use because it necessitates the complicated, expensive control device for the solar reflecting means.
One example of conventional solar following sensors for use in the solar system of the solar indirect-lighting type as noted above is illustrated in FIG. 1. This solar following sensor consists of a base plate 1, a light guide square pillar 2 stood on the base plate 1 and provided in its four side surfaces with light guide grooves 3, and photoelectric light sensors 4 disposed on the base plate 1 within the respective light guide grooves 3. In the case where the axis 1a of the square pillar 2 is precisely pointed toward the sun, the sunlight strikes all the light sensors 4 so that the photoelectric outputs from the four light sensors 4 become equal in intensity. On the contrary, if the axis 1a of the square pillar 1 is deviated from the direction to the sun, the photoelectric output from at least one of the light sensors 4 is weakened. By utilizing this phenomenon, the base plate 1 can be easily faced toward the sun. However, as shown in FIG. 2. the brightness Bs of the sunshine and the brightness Br of a bright sky as the background of the sun are over the dynamic sensitivity (characteristics Rs) of a common photoelectric light sensor such as a photo transistor. This means that the difference D in Brightness between the sun and the bright sky is a little and therefore cannot exactly be detected by the photoelectric light sensor. Thus, the conventional solar following sensor makes it difficult to reliably follow the sun because the brightness of the bright sky become noise from which the sunlight cannot be distinguished.